A question about High Density Bipolar- 3 Zeros

Hey guys. Good night to everybody. I'm just trying to understand why the substitution in this picture was made like this(High density bipolar sc
rambling) of zeros:
The rules state that the sequence of 8 zeros that go after a negative pulse and an even number of ones since the last substitution must be: +00+ and what's being done instead at the beginning of the second signal is 000- which corresponds to an odd number of zeros and a negative pulse before the sequence of 8 zeros. This is happening at the beggining of the signal at the bottom of the picture. This also happens in practically all pictures I've seen.
Could anybody tell me why is this done?
Thanks a lot!
I'm sorry if this was the wrong forum to post this.